Since 1947, magnetrons have undergone extensive development directed principally toward their evolution from the very expensive transmitters used in World War II radar to the present low cost and highly reliable units applicable to use in a kitchen appliance. This effort has reduced the magnetrons cost from several hundreds of dollars for the early cooking magnetrons to less than $20.00 wholesale cost for the present high production tubes. This downward trend continues, although at a lower rate, in spite of steadily increasing labor and materials costs. Along with price reduction, great strides have also been made in compactness and reliability, resulting in greater design freedom and longer life.
Recently, however, because of world affairs impacting the cost of cobalt metal used in the magnet circuit of most magnetrons, much development effort has been expended in reducing the amount of magnetic material required, and hence the cost, of magnetrons using permanent magnets. A substitute for cobalt bearing magnets has been found in ferrite materials, but these have thermal temperature cooefficients almost one order of magnitude (nine times) greater than the cobalt based materials and so are not stable in microwave ovens which have high or changing ambient temperatures such as those using resistance heated browning elements. In addition, ferrite magnets have greater bulk and are subject to cracking due to temperature gradients.
It has become well known that the efficiency of the magnetic circuit of a magnetron is improved as the magnetic material is moved closer to the working gap, since leakage flux is thereby reduced. Early cooking magnetrons had magnetic circuit figures of merit of only 0.3% but due to the development trend to minimize cobalt costs, this figure has been extended over ten times in the last ten years. Recently, in order to optimize this figure, it has been considered necessary to include the cobalt-containing material (such as the Alnico or samarium-cobalt alloys) within the vacuum envelope of the magnetron. This practice has dissadvantages in that the outgassing (pumping) time of the device is considerably increased with attendant cost increase.
Another dissadvantage of the conventional magnetron design is that the magnetic pole pieces adjacent to either end of the electron emitting filament are at positive potential with respect to the electron emitter. The result is that some electrons are therefore attracted axially to the positive pole pieces, reducing the efficiency of the magnetron and also causing excess noise generation during the build-up of coherent oscillations. Such noise is very broad spectrally, causing interference to other services such as televisin, microwave communications, and radio receivers. To prevent the radiation of such noise components, as required by governmental agencies, necessitates the addition of radiation filter components which add appreciably to the manufacturing cost of the magnetron, as well as to its bulk.
Another problem with the prior art magnetron structure is that the filament is cantilever supported from one end and constitutes an assembly having two distinct mechanical resonances the vibrating masses of which are joined by the thoriated tungsten emitting material which, after conversion to tungsten carbide, in the process known as carburizing, is extremely brittle. As a consequence, any mechanical shock or vibration during shipping or rough handling, subjects the filament to stresses which often result in breakage. Also, since the filament and both end hats and the center rod operate at very high temperatures, they must be made of exotic and expensive metals such as tungsten and molybdenum and joined with expensive materials such as platinum, ruthenium, or rutanium-molybdenum alloys which add greatly to the expense of manufacture. Also, because of the high operating temperatures, cantilever supported filaments are usually restricted to operation only in the vertical position to prevent sagging, or mechanical deformity due to gravity.